The high pressure turbine nozzle of a gas turbine engine performs an aerodynamic function in that it accelerates and directs the hot gas flow from the combustor into the high pressure turbine rotor. As such, the turbine nozzle experiences large pressure loads due to the reduction in static pressure between the inlet and exit planes. It also experiences high thermal gradients resulting from the interfacing of hot turbine gases and coolant air at the turbine nozzle. The support structure of the turbine nozzle reacts to the pressure loads at the inner and outer flowpath diameters. These loads are transferred out of the nozzle through the cold structures into the engine casings and frame.
The turbine nozzle, which is commonly constructed of nozzle segments of paired vanes, is typically attached by bolts or a combination of bolts and a clamping arrangement to the inner support structure. At the outer flowpath interface, no mechanical retention mechanism is typically used but the pressure load across the nozzle is relied on to maintain contact with the turbine shroud support.
It is important, for reasons of engine performance, that the inner and outer interfaces both provide good air seals. The pressure drop across these interfaces is of similar magnitude to that across the nozzle itself. Any air leaking across the inner or outer interfaces has not been accelerated to the hot gas stream velocity exiting the turbine nozzle and is therefore a chargeable performance loss to the engine. Modern engines make use of chordal or line seals at these locations to allow relative axial motion of the inner and outer structures while maintaining minimal leakage.
There are several problems in executing all of the above required features. Current designs produce relatively complex nozzle assemblies that require unacceptable amounts of time to assemble and disassemble from turbine engines. This results in a substantial contribution to engine down-time during maintenance overhauls where such nozzle segments must be replaced. Further, increased aircraft engine competition and higher engine operating temperatures has raised the standards for engine reliability and time between overhaul. Improved design nozzles are therefore required to withstand high thermal gradients for increased periods without using excessive cooling airflow.
A need therefore exists for an improved nozzle assembly and mounting arrangement that can be easily assembled and disassembled from a turbine engine.
A further need exists for an improved turbine nozzle capable of operating for extended periods between overhaul in modern high temperature engines.